Normally, alterable carrier frequencies for message transmission are generated using phase locked loops (PLLs).
Such a PLL circuit is shown using a block diagram in FIG. 4. In this case, a phase comparator 1 in a forward path actuates an oscillator 4. The actuation is effected via a charge pump circuit 2 and a low-pass filter 3. The phase comparator in a PLL is also called a phase detector or phase/frequency detector. A feedback path in the PLL contains a frequency divider 8 which divides down the frequency of the oscillator frequency fvco. The phase comparator 1 compares the divided-down frequency fvco with a reference frequency fref. The desired output frequency fvco can be set in such an arrangement by altering the frequency division ratio in the feedback path.
For modern, digital radio systems, carrier frequency generation and digital frequency modulation can be performed using a development of the PLL described, which is called a ΣΔ fractional N PLL. In this case, as FIG. 4 shows, the divider 8 in the feedback path is in the form of a multimodulus divider which is actuated by a digital ΣΔ modulator 9. The desired frequency modulation takes place digitally in this case by varying the frequency division value of the multimodulus divider 8.
The prior art document U.S. Pat. No. 6,008,703 describes such a PLL with a ΣΔ modulator. The PLL indicated therein comprises, in the forward path, a phase/frequency detector which uses a loop filter to actuate a voltage-controlled oscillator. The feedback path contains a multimodulus divider which is actuated by a digital ΣΔ modulator. The ΣΔ modulator, in turn, is supplied firstly with information about the desired carrier frequency and secondly with conditioned and filtered digital modulation data.
When stipulating the dimensions of or designing such a phase locked loop, the choice of bandwidth for the phase locked loop is of particularly great importance. In this context, it is necessary to find a compromise between noise properties and modulation bandwidth. On the one hand, the noise needs to be as low as possible, for example in order to adhere to the spectral transmission masks prescribed in, for example various radio specifications. This requires the selection of a relatively small loop bandwidth. On the other hand, this is opposed by the fact that transmitting modulated data requires a large bandwidth for modern applications in communication technology.
A system-related, dominant noise component is produced as a result of the quantization noise of the ΣΔ modulator itself. The ΣΔ modulator normally actuates the multimodulus divider and in so doing brings about random changeover between integer division ratios.
A multimodulus divider which, as in the prior art patent U.S. Pat. No. 6,008,703, comprises a series circuit containing a plurality of two/three frequency dividers affords a set of integer division values based on the specification
  N  =            N      o        +                  ∑                  i          =          0                          L          -          1                    ⁢                          ⁢                        c          i                ·                  2          i                    where L=number of two/three divider stages and N0=2L.
In this case, a two/three divider is understood to mean a frequency divider whose frequency division ratio can be changed over between the division values 2 and 3. The control lines which the modulator uses to actuate the multimodulus divider are in this case denoted by the range CL-1 to C0. In the case of the described prior art, the more or less random changeover of the division factor by the ΣΔ modulator when the division value is varied results in a minimum step size for the division factor of ΔN of 1. However, this more or less random changeover of the division factor also brings about a change in the frequency over time and therefore produces a “frequency or phase interference swing”.
The ΣΔ modulator distributes the power of this quantization noise in line with its noise transfer function over the frequency band. The quantization noise is raised from low frequencies to higher frequencies as a result. This is also called residual FM jitter, or else phase noise. The magnitude of this interference swing determines the signal-to-noise ratio (SNR) of the frequency-modulated or phase-modulated carrier signal and therefore has a significant effect on the system properties of such a transmitter, such as its adjacent channel interference.
Difficulties may also arise as a result of the aforementioned demanded adherence to a spectral transmission mask in practically all telecommunication standards.
The aforementioned prior art patent U.S. Pat. No. 6,008,703 attempts to solve the problems described by designing the bandwidth of the control loop to be significantly smaller than is actually required for transmitting the modulated data. To compensate for the resultant frequency response of the loop filter, the data to be modulated are first of all subjected to digital precompensation using a filter before being fed into the ΣΔ modulator. This involves high frequency components being raised digitally. A fundamental drawback of this solution is its required, highly accurate alignment between the digital filter for compensation and the analog loop filter.
By way of example, the analog loop filter is subject to temperature drift effects, ageing influences, manufacturing tolerances etc. If these alter the bandwidth of the control loop, the digital precompensation causes the higher frequency components to be raised too much or too little.
Another conventional option for compensating for the reduced loop bandwidth is provided by “two-point modulation”. This involves the modulation data being fed into the phase locked loop at two different input points. This is normally done firstly via the frequency divider and secondly at the input of the controlled oscillator. In this case, the modulation point on the frequency divider has low-pass filter properties, while the analog modulation point at the oscillator input has high-pass filter properties. The overall result is therefore a constant transfer function for the modulation data. However, two-point modulation also has the problem of avoiding mismatches between the analog and digital signal paths. Worded conversely, two-point modulation has very high demands for matching between the analog and digital signal paths.
The prior art patent U.S. Pat. No. 6,424,192 B1 shows a fractional N PLL with a multiple feedback VCO to which a multiplexer is connected. This allows the reference frequency to be increased for the same channel allocation and therefore allows the phase noise of the VCO to be reduced.